A Study on Two-Step Acid Pickling in Trivalent Chromium Process for Aluminum Alloys

• Main Authors: Hsing-Ying Tsai, 蔡幸穎
• Other Authors: 顏秀崗
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/8j296x

碩士 === 國立中興大學 === 材料科學與工程學系所 === 106 === In the aerospace industry, a chromate conversion coating (CCC) is generally used in the surface treatment for aluminum alloys due to its excellent ability to resist corrosion and to promote paint adhesion. However, since the hexavalent chromium compound may cause cancer to the human and is harmful to the environment, to replace the chromate conversion coating, several alternative processes have been developed. The trivalent chromium process (TCP) is the most widespread process in all environment-friendly replacements. In this study, it was investigated how the acid pickling pre-treatment influences the anti-corrosion ability of commercial trivalent chromium treating solution CHEMEON TCP-HF. During the process of acid pickling, the smut, which was formed on surface after heat treatment was removed in order to make the surface favorable for the deposition of trivalent chromium conversion coating and consequently pass the aerospace salt, spray testing standard. The acid pickling effects of two different solutions, Bonderite C-IC Deoxidizer 6MU Acid Deoxidizer Aero (hereinafter called Deox #6)/ nitric acid mixture and 50% nitric acid, on the formation of TCP coating were compared in this research. SEM, TEM and XPS were used for characterization to help figure out the mechanisms of two different acid pickling methods and optimize the related process parameters. The results proved that the smut layer on surface was removed by Deox #6/ nitric acid mixture, but the immersion in Deox #6/ nitric acid mixture also resulted in pitting on aluminum matrix, and thus the salt spray test was failed. On the other hand, though numbers of pitting were greatly decreased by the acid pickling with 50% nitric acid; the smut and oxide remained because of its low etching rate. Given the reasons above, the two-step acid pickling, that is, immersing into Deox #6/ nitric acid mixture for acid pickling at first, and then activating surface with 50% nitric acid, was found to be effective in both the removal of smut and the activation of passivated layer. Besides, by the two-step acid pickling, copper deposition on aluminum alloy surface would be dissolved so that aluminum matrix would be reacted with the conversion coating solution sufficiently, and therefore, the protective layer that passed the 96-hour salt spray test and successfully resisted the localized corrosion was formed.